The crystal structure of the selenide-based synthetic sulfosalt CuPbSb3Se6

Single crystals of copper lead triantimony hexaselenide were obtained as a minor phase during systematic studies of the formation conditions of selenide-based sulfosalts. The crystal structure is an unusual representative of the family of sulfosalts. Instead of the expected galena-like slabs with octahedral coordination, it features mono and double-capped trigonal–prismatic, square-pyramidal and trigonal–bipyramidal coordination.


Chemical context
Sulfosalts (Moë lo et al., 2008) are promising candidates as thermoelectric materials owing to their high electrical conductivity paired with a low thermal conductivity. Inspired by natural sulfur-based sulfosalts, we attempted to further increase the electrical conductivity by substituting Se for S. During systematic studies of the formation conditions of sulfosalts of the andorite structure type (Moë lo et al., 2008), we obtained crystals of the title compound, CuPbSb 3 Se 6 , as a minor phase, by heating the precursor selenides Cu 2 Se, PbSe and Sb 2 Se 3 in evacuated fused silica ampules. Surprisingly, the title compound does not follow the expected crystal chemistry of the structural family. In fact, crystals of the andorite family are modular structures, which are composed of galena-like slabs, with octahedral coordination of the metal atoms. This coordination is not observed for CuPbSb 3 Se 6 . Nevertheless, certain structural relationships can be established, as will be shown below. These structural relationships are reflected by andorite-like compounds of the Sn 3 Bi 2 Se 6 structure type (Chen & Lee, 2010) with very similar cell parameters yet a different space-group symmetry. The structure with the closest matching cell parameters is SnPb 2 Bi 2 S 6 (Li et al., 2019) with a = 20.5458 (12) Å , b = 4.0925 (4) Å and c = 13.3219 (10) Å , whereby the axes have been cyclically permuted with respect to the cell of CuPbSb 3 Se 6 presented here. SnPb 2 Bi 2 S 6 crystallizes in a lillianite-type 4 L (Moë lo et al., 2008) structure and was investigated by the authors for its thermoelectric performance, sporting a figure of merit ZT of 0.3. Since CuPbSb 3 Se 6 shows strongly disordered positions, it is possible that it exhibits similar thermoelectric properties.
It should be noted that from a structural point of view, lillianites and andorites are interchangeable terms. However, in a mineralogical context, they define distinct sulfosalt mineral groups because the Sb that replaces Bi from the lillianite structure in andorite forms electron-pair micelles that distort the structure (Makovicky & Topa, 2014).

Structural commentary
Crystals of the title compound crystallize in the Pnnm space group. All atoms are located on or disordered about (in the case of Sb4A) the reflection plane parallel to (001), which corresponds to the Wyckoff position 4g. The crystal structure is comprised of three mixed Pb/Sb positions, one Sb and one Cu position (Fig. 1). There are three different kinds of coordination polyhedra, with the interatomic distances compiled in Table 1. The predominantly Pb Pb1/Sb1 position is coordinated by Se atoms, forming a double-capped trigonal prism. The predominantly Sb Sb2/Pb2 and Sb3/Pb3 positions and the disordered Sb4/Sb4A are quadratic pyramids in the case of Sb and mono-capped trigonal prisms in the case of Pb. Finally, the disordered Cu1/Cu1A position features trigonalbipyramidal coordination. Whereas the [PbSe 8 ] doublecapped trigonal prisms of the Pb1/Sb1 position are a defining feature of lillianite-type structures and form where the galenalike slabs meet, the remaining two coordinations are unexpected in this structural family.
It has to be noted that the description of the coordination polyhedra of the Sb2/Pb2, Sb3/Pb3 and Sb4/Sb4A positions as quadratic pyramids and capped trigonal prisms is not completely unambiguous. Both variants based on the central atom are shown in Fig. 2 for Sb3 and Pb3. Since the distance from Sb3 to the two farther Se6 atoms is 3.7015 (19) Å and the corresponding calculated bond valence, using the parameters R 0 = 2.60 Å and b = 0.37, is only 0.05, they are considered not to coordinate with Sb3. In contrast, the Se2 and Se3 atoms at the base of the pyramid are located at 2.9293 (15) Å and 2.8835 (14) Å , respectively. The Se1 atom at the apex of the pyramid is located at 2.587 (2) Å from the Sb3 atom. This is different for Pb3, where the two distant Se6 atoms are much closer, with the atomic distances changed to 3.35 (3) Å . The other Se atoms are further away with 3.05 (3) Å for Se2, 2.86 (2) Å for Se3 and 3.02 (4) Å for Se1. Note that the large standard uncertainties (s.u.s) of the Pb-Se distances here are due to Pb3 being a minor position in close proximity to Se3. Thus, in the case of the Pb3 atoms, the coordination is clearly a

Figure 2
Graphical comparison of the two different coordination polyhedra of Sb3 and Pb3. Colours as in Fig. 1.
capped trigonal prism, whereas for Sb3 it is better described as quadratic pyramidal. When considering the electron lone-pair of the Sb III atoms, the coordination might also be seen as 1octahedral.
As for the other discussed coordination polyhedra, one might also see the double-capped trigonal prisms that surround the Pb1/Sb1 position (Fig. 3) as quadratic pyramids in the case of Sb because the metal atoms do not lie in the centre of the polyhedron. If the Sb1 atom is realized, one might rather think of a fivefold instead of an eightfold coordination, again with the atoms forming a quadratic pyramid. Here, the bond distances involving the Pb1 atom are 3.0553 (15) Å and 3.1205 (17) Å for the quadratic base (Se1 and Se5) and 2.9124 (14) Å to the apex (Se2). The two Se3 atoms are located at 3.5754 (15) Å from the Pb1 atom and the last Se4 atom, which forms the second cap of the prism at a distance of 3.4061 (16) Å . The coordination of Sb1 is very similar [distance to Pb1 = 0.26 (2) Å ], with a slightly more pronounced quadratic pyramidal coordination.
The (double-)capped trigonal prism is, as stated above, a defining structural element of the lillianite family. It is interesting to note that whereas the 90 angles of all the prisms are perfectly realized owing to the ..m reflection plane, the triangular bases deviate significantly from an ideal trigonal symmetry. The prism around Pb1/Sb1 is formed from a triangle with 48.31 (2) The coordination polyhedron of Pb1/Sb1. Colours as in Fig. 1

Figure 4
Comparison of four-polyhedra-long chains delimited by the Pb1/Sb1 position in (top) CuPbSb 3 Se 6 and (bottom) the lillianite 4 L-type structure of SnPb 2 Bi 2 S 6 . Colour codes: Bi red, Pb grey, Bi/Sn pink, S yellow. other prisms are closer to regular, with the angles deviating the most from 60 being 54.53 (3) for Pb2 and 65.57 (3) for Pb3.
Finally, the trigonal-bipyramidal coordination of Cu is unusual as Cu is usually encountered as coordinated tetrahedrally or in a planar square. This is still somewhat true for Cu1/Cu1A, as the disordering takes place over the trigonal base of the pyramids, placing them both in their own tetrahedron. However, the position closer to the base (Cu1), i.e. with the more trigonal-bipyramidal-like coordination, has a higher occupancy [59.5 (17)%] than the position further removed from the centre of the trigonal bipyramid (Cu1A).
Despite the clearly different coordination polyhedra, CuPbSb 3 Se 6 can nevertheless be described as a distorted 4 L andorite-type structure, since there are four polyhedra between two double-capped trigonal prisms as shown in Fig. 4. However, the spatial distribution of the Se/S atoms is fundamentally different, leading not only to different coordination polyhedra, as described above, but also an altered connectivity of the polyhedra.

Database survey
No compounds containing only copper, lead, antimony and selenium have been deposited in the Inorganic Crystal Structure Database (ICSD; Bergerhoff & Brown, 1987) as of Fall 2022.

Synthesis and crystallization
40.0mg of Cu 2 Se, 47.6mg of PbSe and 125mg of Sb 2 Se were mixed thoroughly and transferred into a fused silica ampoule, which was sealed under vacuum. The ampoule was heated at 1223 K for 2 h, cooled to 873 K over 7 h and held at that temperature for 149 h. After cooling to 473 K over 5 h and quenching in air, the ampoule was opened and the obtained ingot crushed. Among other phases in the andorite family, single crystals of the title compound CuPbSbSe 3 were isolated.